1. Field of the Invention
The present invention relates generally to a system and method for controlling a damping coefficient of a shock absorber (also called a damper, but hereinafter referred to as the shock absorber) interposed between an unsprung mass and sprung mass of the vehicle. The shock absorber is provided with changing means for changing the damping coefficient thereof at multiple stages in response to a control signal.
2. Description of the Background Art
A Japanese Patent Application First Publication No. Showa 61-163011 published on Jul. 23, 1986 exemplifies a previously proposed electronically controlled shock absorber.
In the previously proposed electronically controlled shock absorber, a sprung mass velocity and a relative velocity between the sprung mass and unsprung mass are derived. Then, when a direction of the sprung mass velocity (upward is plus + and downward is minus -) is coincident with a direction of the relative velocity described above (an extension stroke side is + and a constriction (compression) stroke side is -), the shock absorber is controlled so as to provide a relatively high damping coefficient. On the contrary, when both of the directions described above are not coincident with each other, the shock absorber is controlled so as to provide a relatively low damping coefficient.
However, in a case where a large input magnitude from a road surface on which the vehicle runs occurs, a stroke quantity of the shock absorber at the extension stroke side becomes remarkably increased. When the extension stroke side of the shock absorber is reached to an extension stroke limit, the direction of the sprung mass velocity is upward (+) and the direction of the relative velocity between the unsprung mass and sprung mass is toward the extension stroke limit (-). Therefore, since both directions are not coincident with each other, the damping coefficient of the shock absorber is controlled to provide the relatively low damping coefficient. Consequently, the shock absorber extends toward the extension stroke limit and generates an abnormal sound and vibrations thereat.